The operating characteristics of high pressure sodium lamps are determined by the pressure and composition of the discharge produced in the lamp. As is known, the discharge in the high pressure sodium lamps contains sodium, mercury and xenon, which have the following characteristic pressures in operation: 10.sup.4, 10.sup.5 and 3.times.10.sup.4 Pa, respectively. An example of a typical high pressure sodium lamp can be found in U.S. Pat. No. 3,906,272 issued to Collins et al on Sep. 16, 1975 and assigned to the same assignee as the present invention. The required vapor pressures of sodium and mercury are typically ensured by a sodium-mercury amalgam with a weight ratio of 1 to 3. Upon excitation of the fill contained within the arc tube, radiation in the visible spectrum occurs. The useful radiation is for the amalgam with a weight ratio of 1 to 3. The useful radiation is for the greatest part provided by the sodium, whereas mercury has the role of increasing the voltage at the lamp terminals, thereby reducing lamp current and making current feedthroughs to be designed easier.
One of the significant factors resulting in the market popularity of high pressure sodium lamps is their long life. This long life characteristic, however, is limited by the voltage rise at lamp terminals during operation. The cause of this voltage rise is the reaction between the sodium content of the lamp and one or more components of the discharge vessel, due to which process sodium will be eliminated from the discharge over time. This effect is the so-called sodium loss that decreases the molar fraction of sodium in the sodium-mercury amalgam which, in turn, alters the sodium pressure in the discharge.
At constant temperature, sodium loss reduces the pressure of sodium which is a minor problem in itself. However, at the same time, mercury pressure increases, resulting in a greater slope in pressure relative to the increase of its molar fraction. This latter change will cause the voltage at lamp terminals to rise which will finally result in the lamp being extinguished. The above facts are well known to those proficient in the field.
It is not surprising, therefore, that several attempts have been made to solve the above problem. The state-of-the-art methods are based on the presumption that the speed of the above-mentioned sodium loss is to be slowed.
One of the approaches is aimed to reduce the speed of the chemical reaction between the discharge tube wall and sodium, and is described in a study titled "The Surface Structure of Translucent Alumina, A Scanning Electron Microscopy Investigation" by A. J. H. M. Kock (Proceedings of the Symposium on High Temperature chemistry II, p: 194-205).
Another attempt was to eliminate or reduce the contact of liquid-phase amalgam with the wall as seen in the disclosures of No. GB 2 072 939 and No. HU 181 782 Patent Specifications.
All the disclosed approaches--two of which were mentioned as examples only--have proven to be more or less successful, but have been unable to solve the problem of sodium loss. This indicates that the importance of the various factors affecting the process is still not clear.